Electrochemical process for oxidation of alkanes to alkenes

ABSTRACT

An electrochemical process for the oxidation of an alkane to at least one corresponding alkene uses an electrochemical cell having an anode chamber on one side of a proton conducting medium, and a cathode chamber on the other side of the said medium. The alkane is oxidized in the anode chamber to produce at least one corresponding alkene and protons are transferred through a proton conducting membrane to the cathode chamber where protons combine with a proton acceptor, while generating electricity and water. An apparatus for use in the process is also provided.

FIELD OF THE INVENTION

[0001] This invention relates to an electrochemical process foroxidation of an alkane to the corresponding alkene using anelectrochemical cell that has a proton-conducting medium. The process ofthe invention is for the selective production of alkenes whilegenerating electricity and water.

BACKGROUND OF THE INVENTION

[0002] An alkane can be converted to the corresponding alkene by severalprocesses, including partial oxidation and thermal cracking. Accordingto these processes, for example, propane may be converted to propylene.Other alkanes can also be similarly converted to a corresponding alkene,for example: butane to one or more of 1-butene and 2-butene, and ethylbenzene to styrene.

[0003] Propane can be chemically oxidized to a mixture of productsincluding propylene by reaction with a limited amount of oxygen.Catalysts are known for the activation of propane. When a mixture ofpropane and a limited amount of oxygen is passed over a catalyst amixture of products is formed, including propylene, other hydrocarbonproducts, and oxides of carbon. It is very difficult to oxidize propaneselectively to propylene. Typically, when propane is heated to a hightemperature, typically several hundreds of degrees Celsius, the propaneis cracked to form a mixture containing hydrogen, propylene, ethane,methane, ethylene, and higher hydrocarbons. The cracking processconsumes energy. Further, the cracking process is not highly selectiveto propylene, and typically operates at low conversion. It is thereforenecessary to separate the products of a catalytic oxidation reaction toobtain propylene in a commercially saleable or useable form (e.g., withother reaction products of the cracking process removed of reduced on avolume percent). Further, the heat generated by the oxidation reactionis recoverable only as process energy and not as high-grade energy suchas electricity.

[0004] When a fuel is oxidized in a fuel cell, the products are theoxidation products from the fuel and electrical energy. Oxide ionconducting solid membranes are used in solid oxide fuel cells (a“SOFC”). In such cells, a source of oxygen is fed to a cathode catalystwhere the oxygen combines with electrons to form oxide ions. The oxideions pass through the solid membrane from the cathode to the anode. At acatalytic anode in a SOFC, oxide ions react with a fuel to generateoxidation products and electrons. When the fuel is propane, theoxidation products are usually oxides of carbon. Thus an oxide ionconducting SOFC can be designed to use propane as fuel. Mazanec et al.in U.S. Pat. No. 4,933,054 which issued in 1990, describe anelectrochemical process using oxide ion conducting SOFC at temperaturesin the range of about 500° C. to about 950° C. for electrochemicaloxidative dehydrogenation of saturated hydrocarbons. The saturatedhydrocarbons have from 2 to 6 carbon atoms, and include propane, and areconverted to the corresponding unsaturated hydrocarbons, includingpropylene. Michaels and Vayenas, in Journal of Catalysis, Volume 85,477-487 (1984), describe electrochemical oxidative dehydrogenation ofethyl benzene to styrene in the vapor phase using SOFC operated at hightemperatures (e.g. above 650° Celcius).

[0005] Proton conducting solids are known, including polymer electrolytemembranes (“PEM”). PEM are used in H₂—O₂ fuel cells, an example of whichis as described by Fuglevand et al. in U.S. Pat. No. 6,030,718. Thehydrogen used as fuel in PEM fuel cells can be generated in severalways. Propane can be reformed to generate a hydrogen containing fuel fora fuel cell, and can be used as a coolant for a fuel cell. For example,Ziaka and Vasileiadis in U.S. Pat. No. 6,090,312, issued in 2000,disclose reforming reactions of light hydrocarbons having from 1 to 4carbon atoms to generate hydrogen for use as fuel in a fuel cell.Nakagaki et al. in U.S. Pat. No. 6,099,983, issued in 2000, disclosesreforming of propane to generate a hydrogen containing gas that is usedas fuel in a fuel cell, in which the reformed hydrogen containing gasalso serves as coolant for the fuel cell. Each of the above examplesuses propane as a source of hydrogen to be used as fuel, and does notuse propane as fuel.

SUMMARY OF THE INVENTION

[0006] One aspect of the present invention relates to operating analkane fuel cell with a proton conducting medium that converts an alkaneto at least one corresponding alkene at low temperatures and preferablylow pressures.

[0007] Another aspect of the present invention relates to operating analkane fuel cell with a proton conducting medium that converts an alkaneto the corresponding alkene with a high degree of selectivity.

[0008] In accordance with one embodiment of this invention, there isprovided an electrochemical process for oxidation of an alkane to acorresponding alkene using an electrochemical cell having an anodechamber having an anode and a cathode chamber having a cathode, theanode chamber and the cathode chamber separated at least in part by aproton conducting medium, said process comprising:

[0009] (a) providing at least one alkane to the anode chamber;

[0010] (b) providing an oxygen containing gas to the cathode chamber;

[0011] (c) passing protons through the said medium from the anodechamber to the cathode chamber

[0012] whereby at least a portion of the alkane is converted to acorresponding alkene.

[0013] In one embodiment, the anode comprises at least one metalcatalyst active for activation of the alkane and the anode and cathodeare in electrical contact with each other and the process comprisesproducing electrons during the conversion of the alkane to the alkeneand the catalytic cathode comprises at least one metal catalyst activefor combination of oxygen with protons and electrons to form water.

[0014] In another embodiment, the process further comprises maintainingthe electrochemical cell at a temperature and a pressure that maintainsthe moisture of said medium.

[0015] In another embodiment, the process further comprises providingthe alkane in a gaseous state.

[0016] In another embodiment, the alkane is selected from the groupconsisting of propane, a mixture of propane and at least one inert gas,a mixture of propane and at least one inert liquid, and a mixture ofhydrocarbons containing propane, and the process comprises producingpropylene as the corresponding alkene.

[0017] In another embodiment, the alkane is selected from the groupconsisting of butane, a mixture of butane and at least one inert gas, amixture of butane and at least one inert liquid, and a mixture ofhydrocarbons containing butane, and the process comprises producing atleast one of 1-butene and 2-butene as the corresponding alkene.

[0018] In another embodiment, the alkane is selected from the groupconsisting of a mixture of ethyl benzene and at least one inert gas, amixture of ethyl benzene and at least one inert liquid, and a mixture ofhydrocarbons containing ethyl benzene, and the process comprisesproducing styrene as the corresponding alkene.

[0019] In another embodiment, the oxygen containing gas is selected froma group consisting of oxygen, a mixture of oxygen and at least one inertgas, and air and the process further comprises combining protons whichhave passed through the medium and oxygen to produce water.

[0020] In another embodiment, the process is operated at a temperatureof at least about 50° C.

[0021] In another embodiment, the, process is operated at a temperaturein the range of about 50° C. to about 155° C.

[0022] In another embodiment, the process is operated at a temperaturein the range of about 50° C. to about 100° C.

[0023] In another embodiment, the process is operated at a pressure ofat least atmospheric pressure and below a pressure at which one or moreof the alkane and the alkene will condense to form a liquid phase.

[0024] In another embodiment, the pressure is maintained sufficientlyhigh so as to maintain moistness of the proton-conducting medium.

[0025] In another embodiment, the process is operated at a pressure ofat least atmospheric pressure and below a pressure at which one or moreof propane and propylene will condense to form a liquid phase, thepressure being sufficiently high so as to maintain the moistness of theproton conducting medium at the operating temperature.

[0026] In another embodiment, process is operated at a pressure of atleast atmospheric pressure and below a pressure at which one or more ofbutane, and at least one of 1-butene and 2-butene will condense to forma liquid phase, the pressure being sufficiently high so as to maintainthe moistness of the proton conducting medium at the operatingtemperature.

[0027] In another embodiment, the process is operated at a pressure ofat least atmospheric pressure and below a pressure at which one or moreof ethyl benzene, and styrene will condense to form a liquid phase, thepressure being sufficiently high so as to maintain the moistness of theproton conducting medium at the operating temperature.

[0028] In another embodiment, the process is operated at a pressure inthe range of about 0.5 atm to about 10 atm.

[0029] In another embodiment, the process is operated at aboutatmospheric pressure.

[0030] In accordance with another aspect of the instant invention, thereis provided an electrochemical apparatus for oxidation of an alkane to acorresponding alkene comprising:

[0031] (a) an anode chamber having an anode, the anode comprising ametal catalyst active for activation of the alkane;

[0032] (b) a cathode chamber having a cathode, the cathode comprising ametal catalyst active for combination of a proton acceptor with protons;and,

[0033] (c) a proton conducting medium positioned in fluid flowcommunication with both the anode chamber and the cathode chamber.

[0034] In one embodiment, the proton acceptor comprises oxygen.

[0035] In another embodiment, the proton acceptor is a gas selected froma group consisting of oxygen, a mixture of oxygen and at least one inertgas, and oxygen is combined with protons that have passed through themedium and oxygen to produce water.

[0036] In another embodiment, the alkane is gaseous.

[0037] In another embodiment, the alkane is a linear molecule or alinear substituent of a cyclic or aromatic molecule.

[0038] In another embodiment, the alkane has a carbon chain length offrom 2 to 6 carbon atoms.

[0039] In another embodiment, the proton conducting medium is a solidperfluorosulphonic acid proton conducting membrane.

[0040] In another embodiment, the catalytic anode and the catalyticcathode separately are formed of compressed carbon powder loaded withmetal catalyst, the metal catalyst of the catalytic anode being selectedfrom metal catalysts active for activation of an alkane, and the metalcatalyst of the catalytic cathode being selected from metal catalystsactive for combination of oxygen with protons and electrons to formwater.

[0041] In another embodiment, the alkane comprises propane and thecatalytic anode and the catalytic cathode separately are formed ofcarbon cloth loaded with metal catalyst, the metal catalyst of thecatalytic anode being selected from metal catalysts active foractivation of propane, and the metal catalyst of the catalytic cathodebeing selected from metal catalysts active for combination of oxygenwith protons and electrons to form water.

[0042] In another embodiment, the alkane comprises butane and thecatalytic anode and the catalytic cathode separately are formed ofnickel mesh impregnated with metal catalyst, the metal catalyst of thecatalytic anode being selected from metal catalysts active foractivation of propane, and the metal catalyst of the catalytic cathodebeing selected from metal catalysts active for combination of oxygenwith protons and electrons to form water.

[0043] In another embodiment, the metal catalyst active for activationof alkane is selected from the group consisting of platinum, palladium,silver, nickel, cobalt, gold, bismuth, manganese, vanadium, ruthenium,copper, zinc, chromium, iron or indium oxide-stannous oxide mixtures, orany mixtures thereof.

[0044] In another embodiment, the metal catalyst active for activationof the alkane is selected from the group consisting of nickel, cobalt ora mixture of nickel and cobalt.

[0045] In another embodiment, the metal catalyst for activation ofalkane is selected from the group consisting of platinum, palladium or amixture of platinum and palladium.

[0046] In another embodiment, the metal catalyst active for combinationof oxygen with protons and electrons to form water is selected from thegroup consisting of nickel, cobalt, gold, bismuth, manganese, vanadium,ruthenium, copper, zinc, chromium, iron or indium oxide-stannous oxidemixtures, or any mixtures thereof.

[0047] In another embodiment, the metal catalyst active for combinationof oxygen with protons and electrons to form water is selected from thegroup consisting of nickel, cobalt or a mixture of nickel and cobalt.

[0048] In another embodiment, the metal catalyst active for combinationof oxygen with protons and electrons to form water is selected from thegroup consisting of platinum, palladium or a mixture of platinum andpalladium.

[0049] In another embodiment, the apparatus is operated at a temperatureof at least about 50° C.

[0050] In another embodiment, the apparatus is operated at a temperaturein the range of about 50° C. to about 155° C.

[0051] In another embodiment, the apparatus is operated at a temperaturein the range of about 50° C. to about 100° C.

[0052] In another embodiment, the process is operated at a pressure ofat least atmospheric pressure and below a pressure at which one or moreof the alkane and the alkene will condense to form a liquid phase.

[0053] In another embodiment, the pressure is maintained sufficientlyhigh so as to maintain moistness of the proton-conducting medium.

[0054] Accordingly, one advantage according to one aspect of the instantinvention is that the conversion of alkanes to alkenes may be conductedat temperatures below 200° C.

[0055] Another advantage of the invention is that according to oneaspect, the process may be used to operate a propane fuel cell thatconverts propane with a high degree of selectivity to propylene.

[0056] Another advantage of the invention is that according to oneaspect, the process may be used to oxidize propane selectively topropylene at a temperature lower than a temperature of operation of aSOFC and even at a temperature below thee boiling point of water, andthereby recover water as liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] These and the other features of the invention will become moreapparent from the following description in which reference is made tothe appended drawings, wherein:

[0058]FIG. 1 is a schematic diagram of a cell for the electrochemicaloxidation of an alkane to the corresponding alkene.

[0059]FIG. 2 is a diagram showing the electrochemical reactionscomprising the electrochemical oxidation of propane to propylene and theelectrochemical reduction of oxygen to water in the fuel cell of FIG. 1.

[0060]FIG. 3 is a diagram comprising three plots of the-relationshipbetween current and potential for operation of a laboratory scaleversion of the cell for the electrochemical oxidation of propane topropylene illustrated in FIG. 2, using different compositions of thecatalytic anode at different operating temperatures and pressures.

[0061]FIG. 4 is a diagram showing the relationship between current andpotential for operation of a laboratory scale version of the cellillustrated in FIG. 1 for the electrochemical oxidation of butane tobutene at atmospheric pressure and 90° C.

[0062]FIG. 5 is a diagram showing the relationship between current andpotential for operation of a laboratory scale version of the cellillustrated in FIG. 1 for the electrochemical oxidation of ethyl benzeneto styrene at atmospheric pressure and 90° C.

DETAILED DESCRIPTION OF THE INVENTION

[0063] The electrochemical cell and process for electrochemicaloxidation of an alkane to one or more corresponding alkenes will now bedescribed with reference to FIGS. 1 through 5. The process of thisinvention is applicable to any alkane. The alkane may be a linear alkane(e.g. propane and butane) or a cyclic or aromatic alkane (e.g.cyclohexane, ethyl benzene). The linear alkane may have a straight chainor may be branched. Preferably, the alkane is a linear alkane or asubstituted cyclic or a substituted aromatic alkane (e.g.tetrahydronaphthalene). In the case of a substituted cyclic or asubstituted aromatic alkane, the reaction is preferably targeted atconverting the substituant carbon chain and not the cyclic or aromaticportion of the molecule. For example, the organic feedstock may be2-methyl alkane so that, e.g., 2-methylhexane could be converted to2-methyl-1-hexene plus the 2-methyl-2-hexene isomer. More preferably,the alkane is a linear alkane. Most preferably, the alkane is a linear,non-branched alkane. The alkane may have a chain length of from 2 to 12carbon atoms and preferably from 2 to 6 carbon atoms.

[0064] The alkane may be in any form that can flow so as to flow throughthe electrochemical cell. The alkane may be a liquid or a gas. If thealkane is a light hydrocarbon, then the organic feedstock is preferablyin the form of a gas since otherwise an elevated pressure would berequired to cause the hydrocarbon to be in its liquid form. Similarly,if the organic feedstock is a mid-range hydrocarbon (e.g. decane) thehydrocarbon is preferably in the form of a liquid since otherwise arelatively high temperature would be required to use the hydrocarbon inits gaseous state.

[0065] According to this process, one or more bonds in the alkane isconverted to an alkene. Thus this process may be used to convert propaneto propylene or to convert butane to 1-butene and/or 2-butene.Preferably, the alkene generated by the process of the present inventionis reagent grade alkene. The reaction has a high degree of selectivity.By this it is meant that a high percentage, and preferably substantiallyall, of the alkane that is consumed by the process is converted to thecorresponding alkene (or if there is more than one corresponding alkene,then to one or more of the corresponding alkenes) and that only a minorportion, and preferably essentially none, of the corresponding alkyne isproduced. Unlike catalytic cracking, the carbon chain length of thealkane in the feedstock is essentially not altered.

[0066] For purpose of illustration only, the process of the presentinvention will be described for the electrochemical conversion ofpropane to propylene. Application of the process for the conversion ofalkane anode feeds, including propane, butane and ethyl benzeneseparately, will then be illustrated by a series of non-limitingexamples.

[0067] The process of this invention may be conducted in anyelectrochemical cell that has at least one anode and at least onecathode that are each in ionic contact with a proton-conducting medium.In one embodiment, the electrochemical cell used in the invention hasone anode, one cathode and a proton-conducting medium.

[0068] In the embodiment of FIG. 1 electrochemical cell 10 has a body 11enclosing an anode chamber 12 on one side 14 of a proton conductingmedium 16, and a cathode chamber 18 on another side 20 of said medium16. Anode chamber 12 has a first inlet 22 and a first outlet 24. Cathodechamber 18 has a second inlet 26 and a second outlet 28. Body 11 and theproton conducting medium 16 are electrically insulated from each otherby insulators 29. A first current collector 40 is in electrical contactwith catalytic anode 32. A second current collector 42 is in electricalcontact with catalytic cathode 34. It will be appreciated that cell 10may be of a variety of configurations provided that protons are producedat least one anode chamber and transmitted to at least one cathodechamber through a proton conducting medium.

[0069] In one embodiment, catalytic anode 32, catalytic cathode 34 andsolid proton conducting membrane 30, which functions as theproton-conducting medium, are assembled as a membrane electrode assembly(MEA). However it will be appreciated that each of these elements may beseparately assembled into a cell and need not be positioned adjacenteach other as shown in FIG. 1.

[0070] The proton-conducting medium can be made from any material thatcan transfer protons from catalytic anode 32 to catalytic cathode 34.Commercially available proton conducting materials suitable for membrane30 include perfluorosulphonic acid polymer available under the trademarkNAFION™ (Du Pont de Nemours and Company), Gore Select™ (W. L. Gore andAssociates) and ion-exchange amide systems. Membrane materials that maybe used as the proton conducting medium 16 include membranes of modifiedperfuorinated sulphonic acid polymer, polyhydrocarbon sulphonic acid andcomposites of two or more kinds of proton exchange membranes can beused. Membranes of polyethylene and polypropylene sulphonic acid,polystyrene sulphonic acid and other polyhydrocarbon-based sulphonicacids (such as membranes made by RAI Corporation, USA) may also be useddepending on the temperature and duration of fuel cell operation.Composite membranes consisting of two or more types of proton-conductingcation-exchange polymers with differing acid equivalent weights, orvaried chemical composition (such as modified acid group or polymerbackbone), or varying water contents, or differing types and extent ofcross-linking (such as cross linked by multivalent cations e.g., Al³⁺,Mg²⁺ and the like) may be used.

[0071] Catalytic anode 32 comprises or consists essentially of a firstmetal catalyst 36. First metal catalyst 36 is selected from metalcatalysts active for activation of an alkane, for example propane.Activation of an alkane is defined as catalyzing the oxidativedehydrogenation of alkanes to the corresponding alkenes. Examples ofmetals and metal oxides useful in activating an alkane include platinum,palladium, silver, nickel, cobalt, gold, bismuth, manganese, vanadium,ruthenium, copper, zinc, chromium, iron or indium oxide-stannous oxidemixtures, or any mixtures of said metals and metal oxides. In apreferred embodiment, the first metal catalyst 36 of the catalytic anode32 is selected from nickel, cobalt, platinum, palladium, a mixture ofplatinum and palladium, or mixtures thereof. In a more preferredembodiment, the first metal catalyst 36 of the catalytic anode 32 isselected from platinum, palladium or a mixture of platinum andpalladium. Generally, the selection of the first metal catalyst may bebased on the activity of the metal for thermo-catalytic processes withthe selected alkane.

[0072] Catalytic cathode 34 may comprise or consist essentially ofsecond metal catalyst 38. Second metal catalyst 38 is selected frommetal catalysts active for combining oxygen with protons and electronsto form water. Examples of metals and metal oxides useful in combiningoxygen with protons and electrons to form water include platinum,palladium, silver, nickel, cobalt, gold, bismuth, manganese, vanadium,ruthenium, copper, zinc, chromium, iron or indium oxide-stannous oxidemixtures, or any mixtures of said metals and metal oxides. In apreferred embodiment, the second metal catalyst 38 of the catalyticcathode 34 is selected from nickel, cobalt, platinum, palladium, amixture of platinum and palladium, a mixture of nickel and cobalt ormixtures thereof. In a more preferred embodiment, the second metalcatalyst 38 of the catalytic cathode 34 is selected from platinum,palladium or a mixture of platinum and palladium.

[0073] Anode 32 may be constructed from first metal catalyst 36.Alternately, anode 32 may comprise a support loaded with a first metalcatalyst 36. Thus, the catalytic metal may be plated on or other wiseassociated with a support. The support may be inert to the reaction(i.e. it does not electrochemically affect the reaction in anode chamber12 or at least does not deleteriously affect the reaction).Alternatively, the support may be a first catalytic metal. Similarly,cathode 34 may be constructed from second metal catalyst 38 or maycomprise a support loaded with a second metal catalyst 38. For example,the support of catalytic anode 32 or catalytic cathode 34 may be formedfrom compressed carbon powder onto which has been deposited (loaded)metal catalyst 36 or 38 respectively, carbon cloths supporting metalcatalyst 36 and 38 respectively or nickel mesh impregnated with metalcatalyst 36 and 38 respectively. The catalytic anode 32 and catalyticcathode 34 can be assembled according to established methods, forexample, by the methods described in U.S. Pat. No. 6,294,068 granted toPetrovic et al. on Sep. 25, 2001.

[0074] In the process of the present invention, the alkane is providedto anode chamber by any method known in the art and may be removedtherefrom by any method known in the art. Preferably, the alkane isgaseous since the viscosity of the liquid to flow through anode chamber12 is less than if the hydrocarbon feed was in the liquid state. Forexample, in the embodiment of FIG. 1, an anode stream containing propaneis fed through first inlet 22 into anode chamber 12 and exits anodechamber 12 through first outlet 24, as indicated by arrows 44. Anodechamber 12 may include one or more inlets 22 and one or more outlets 24.

[0075] The alkane may be fed alone through one or more first inlets 22into anode chamber 12, or as a mixture containing an alkane diluted withone or more inert gases such as nitrogen, helium, neon, argon, krypton,xenon or any other gas, including steam that does not deleteriouslyinterfere with the oxidative dehydrogenation of the alkane. The alkanecan also be mixed with other hydrocarbon gases, including a mixture withmethane. It will be appreciated that the hydrocarbon feedstock fed totreated in anode chamber 12 may comprise a mixture of alkanes so as toobtain a mixture of the corresponding alkenes. In such a case, eachdifferent alkane may not be converted to the same degree. The alkane oralkanes may also be fed through first inlet 22 into anode chamber 12 bythemselves, or with an inert carrier liquid (e.g. nitrogen or argon) orwith a pure (compressed) liquid (e.g. hexane) that can be used at thetemperatures at which electrochemical cell 10 operates.

[0076] A cathode stream which does not deleteriously interfere with thereaction in cathode chamber 18 is fed through one or more second inlets26 into cathode chamber 18 and exits cathode chamber 18 through one ormore second outlets 28, as indicated by arrows 46. In one embodiment,the cathode stream may consist of or include any proton acceptor (amedium that will accept the protons which are transferred to cathodechamber 12). The medium may be a liquid or a gas and is preferably agas. The gas may be a halogen such as chlorine that when combined withprotons would form HCl. Preferably, the cathode stream consists of orincludes oxygen. The stream may contain oxygen alone, air or a mixtureof oxygen and at least one inert gas. The inert gas may be selected fromnitrogen, helium, neon, argon, krypton, xenon or any other gas that doesnot interfere with the reduction of oxygen to water. It will berecognized that the direction of arrows 44 and the direction of arrows46 is shown for purposes of illustration only, and are not to beconstrued as indicating that the anode stream and the cathode streammust necessarily flow in the same direction across respectively firstside 14 and second side 20 of medium 16. For example, the flow may becounter current or concurrently through chambers 12,18.

[0077] The process of the present invention comprises an anode reactionand a cathode reaction. Referring to FIG. 1, in anode chamber 12, analkane is activated to form the corresponding alkene; protons, andelectrons. Protons pass through proton conducting medium 16 from anodechamber 12 to cathode chamber 18. Electrons are collected at catalyticanode 32 by first current collector 40 and are conveyed to catalyticcathode 34 by second current collector 42. First current collector 40and second current collector 42 may be directly connected to each otheror may be connected via an external electrical load (not illustrated) byelectrical leads 64. If employed, the load (which may be a fixed orvariable resistance controller) is used to control the electrical flowto cathode chamber 18. The leads are insulated to prevent electricalcontact with body 11. Electrons combine with protons and oxygen atactive sites of the second metal catalyst 38 of the catalytic cathode 34to form water. The overall process is the oxidation of an alkane to thecorresponding alkene, and the formation of water from oxygen, protonsand electrons.

[0078] In one embodiment, with reference to FIG. 2, propane 50 (C₃H₈) isactivated at active sites of first metal catalyst 36 of catalytic anode32 to form propylene 52 (C₃H₆), protons 54 (H⁺), and electrons 56 (e⁻)according to Equation 1. Protons 54 pass through proton conductingmembrane 30 from catalytic anode 32 to catalytic cathode 34 as indicatedby arrow 58. Protons 54 combine with electrons 56 and oxygen 60 (O₂) atactive sites of second metal catalyst 38 of catalytic cathode 34 to formwater 62 (H₂O) according to Equation 2. The overall process is oxidationof propane 50 to propylene 52 and the formation of water 62 from oxygen60, protons 54 and electrons 56, according to Equation 3. Electrons 56are collected at catalytic anode 32 by first current collector 40 andare conveyed to catalytic cathode 34 by second current collector 42.First current collector 40 and second current collector 42 are connectedto an external electrical load (not illustrated) by electrical leads 64,as shown in FIG. 1, the leads being insulated to prevent electricalcontact with body 11.

C₃H₈→C₃H₆+2H⁺+2e⁻  [1]

O₂+4H⁺+4e⁻→2H₂O   [2]

2C₃H₈+O₂→2C₃H₆+2H₂O [3]

[0079] The process of the present invention may be conducted at atemperature below about 200° C. The lower range of the temperature ispremised upon the reaction kinetics. As the temperature is reduced, thereaction rate decreases. Preferably, the process is conducted at atemperature from about 20° C. to about 155° C., preferably from about50° C. to about 155° C. and most preferably from about 50° C. to about100° C. If the temperature is below about 50° C., then the reactionproceeds relatively slowly. The process may be conducted below thistemperature if the rate of reaction is acceptable. If the process isconducted at a temperature below about 100° C. (i.e. below the boilingpoint of water at atmospheric pressure), then the resultant water willbe liquid. If the process is conducted above atmospheric pressure, then,liquid water may be produced at an even higher temperature. The processmay be operated at higher temperatures provided that cell 10 maintainsit structural integrity at those temperatures. The process of thepresent invention may also be operated wherein the temperature in theanode chamber 12 can be different than the temperature in the cathodechamber 18. Preferably the temperature in anode chamber 12 issubstantially the same as the temperature in cathode chamber 18. The useof a lower operating temperature permits better control of the feedstockand the product and a reduced level of side reactions. A lowertemperature may increase the longevity of some membranes.

[0080] For example, in one embodiment, when cell 10 uses NAFION™ asproton conducting membrane 30, the oxidation of propane to propylene maybe conducted at a temperature in the range of about 65° C. to about 95°C. When the temperature is below 65° C. the rate of the reaction isslow. When the temperature of the reaction is above 95° C. it isnecessary to operate cell 10 at a pressure greater than atmosphericpressure to ensure that NAFION™ membrane 30 does not lose structuralwater, and thereby remains moist and maintains proton-conductingcapability.

[0081] Optionally, means may be provided for humidifying one or both ofchambers 12, 18, such as by humidifying anode chamber stream 44 beforefirst inlet 22 and/or cathode chamber stream 46 before second, inlet 26to provide sufficient water to prevent membrane 16 from drying out athigher operating temperatures. However, membranes that can conductprotons without having to remain moist, may also serve as the protonconducting medium 16 in the present invention and reduce or eliminatethe need to humidify any of the streams.

[0082] The process of the present invention may be conducted at apressure from about 0.1 atmospheres (atm) to about 100 atm, preferablyfrom about 0.5 atm to about 10 atm and more preferably from about 1 atmto about 5 atm. Further, preferably the pressure is at least atmosphericpressure, thereby providing for a high concentration of alkane atcatalytic anode 32 and a high concentration of oxygen at catalyticcathode 34.

[0083] The process of the present invention is preferably operated at apressure below a pressure at which the alkane to be oxidized, thecorresponding alkene or a combination of the alkane and the alkene wouldcondense to form a liquid phase at the temperature of the reaction.Accordingly, the shorter the carbon chain, the higher the preferredpressure may be.

[0084] The pressure in the anode chamber 12 can be different than thepressure in the cathode chamber 18. Preferably the pressure in anodechamber 12 is substantially the same as the pressure in cathode chamber18, thereby reducing or essentially reducing stress on proton conductingmedium 16 and crossover. Crossover is a process where the alkane feedpermeates through proton conducting medium 16 and combines with oxygenon the catalytic surface of the cathode. Crossover lowers the efficiencyof cell 10, reduces performance and generates heat in the fuel cell.Factors which lower the occurrence of crossover include a lower flowrate of the alkane, a lower concentration of the alkane, operation ofthe electrochemical cell at a lower temperature, and minimizing theaccess of the alkane to the proton conducting medium, such as by thedesign of the anode with hydrophobic and hydrophilic regions. It isappreciated by those skilled in the art that an amount of crossover mayoccur without deleteriously affecting the commercial usability of theelectrochemical cell in the present invention. Unbalanced pressures canlead to rupture of the membrane and possible crossover. However, it willbe appreciated that stress on the proton conducting medium 16 during theoperation of the cell as a result of a difference in pressure betweenthe anode chamber and the cathode chamber can be reduced or essentiallyreduced by structurally reinforcing the proton conducting medium 16.

[0085] As the pressure is increased, the concentration of alkane at thecatalyst sites is increased and the flux is increased. Further, atincreased pressures, the propensity for water to evaporate is reduced,and hence this may improve membrane life and activity.

[0086] Operation of cell 10 to oxidize propane to propylene will now beillustrated with reference to FIGS. 1 through 3 by three non-limitingexamples. Further examples will illustrate operation of the cell tooxidize each of butane in Example 4, and ethyl benzene in Example 5. Thedata are in each case for operation of an unoptimized cell 10 design andunoptimized operating parameters. It will be recognized that improvedperformance of cell 10 may be obtained under different operatingconditions using amendments to the design for cell 10 without departingfrom the spirit of the present invention.

EXAMPLE 1

[0087] Laboratory test equipment was constructed including laboratoryscale cell 10. Laboratory scale cell 10 had a MEA 16 having an effectivesurface area of approximately 1 square centimeter for catalytic anode32. Catalytic anode 32 comprised compressed Teflonized carbon powderloaded with platinum as first metal catalyst 36. Catalytic cathode 34comprised the same material as catalytic anode 32. The anode chamberstream comprised a mixture of propane (10% by volume) diluted withnitrogen as an inert diluent. Oxygen was fed into cathode chamber 18.Open circuit potentials up to 555 millivolts were obtained when cell 10was operated at atmospheric pressure and at temperatures in the range50° C. to 95° C. A series of resistances ranging from 0.1 ohms to1,000,000 ohms was applied as an external circuit load across electricalleads 64 of cell 10. Referring to FIG. 3, it was found that the currentand the potential provided by the cell varied with the load, asillustrated by line 70 for operation of cell 10 at atmospheric pressureand at a temperature of 85° C. For example, for a load of 1.0 ohm thepotential was found to be 24 millivolts and the current was 24milliamps. Samples of the anode chamber effluent from first outlet 24were collected into a gas collection cell for use in an infraredspectrometer. Propylene was detected in the infrared spectrum of theanode chamber effluent in amounts corresponding closely to the amountsexpected from the current generated by laboratory scale cell 10.Conversion of the alkane to alkene was about 4-5%. No propyne wasdetected. However, higher conversion rates could be obtained by reducingthe flow rate of the hydrocarbon feedstock. Thus the selectivity topropylene as opposed to propyne by electrochemical oxidation of amixture containing 10% propane was high. The alkene in the effluent fromthe process could be separated from the alkane in the effluent by meansof the different boiling points of the compounder.

EXAMPLE 2

[0088] Pure propane was fed as anode chamber feed to laboratory cell 10having the same catalytic anode 32 and the same catalytic cathode 34 aswere used in Example 1. Oxygen wars the cathode chamber feed. Theoperating pressure in both of anode chamber 12 and cathode chamber 18was 44 psia. (about 4 atm) and the temperature of cell 10 was 135° C.The open circuit potential generated was 464 millivolts. When anelectrical load was connected across electrical leads 64, it was foundthat the current and the potential provided by the cell varied with theload, as illustrated by line 72 in FIG. 3. When the external circuitload was 1.0 ohm, the potential generated was 42 millivolts and thecurrent generated was 42 milliamps. Propylene was detected in theinfrared spectrum of the anode chamber effluent in amounts correspondingclosely to the amounts expected from the current generated by laboratoryscale cell 10; Conversion was about 4-5%. No propyne was detected. Thus,the oxidation of propane to propylene as opposed to propyne wasselective.

EXAMPLE 3

[0089] Pure propane was fed as anode chamber feed to laboratory cell 10having a catalytic anode 32 comprising compressed Teflonized carbonpowder loaded with palladium as first metal catalyst 36 and the samecatalytic cathode 34 as was used in Example 1 and Example 2. Oxygen wasthe cathode chamber feed. The operating pressure in both of anodechamber 12 and cathode chamber 18 was 44 psia and the temperature ofcell 10 was 135° C. The open circuit potential generated was 353millivolts. When an electrical load was connected across electricalleads 64, it was found that the current and the potential provided bythe cell varied with the load, as illustrated by line 74 in FIG. 3. Whenthe external circuit load was 1.0 ohm, the potential generated was 12millivolts and the current generated was 12 milliamps. Propylene wasdetected in the infrared spectrum of the anode chamber effluent inamounts corresponding closely to the amounts expected from the currentgenerated by laboratory scale cell 10. Conversion was about 4-5%. Nopropyne was detected. Thus, the oxidation of propane to propylene asopposed to propyne was selective.

EXAMPLE 4

[0090] Pure butane was fed as anode chamber feed to laboratory cell 10having a catalytic anode 32 comprising compressed Teflonized carbonpowder loaded with 4.2% platinum and supported on carbon cloth as firstmetal catalyst 36. The catalytic cathode was a similar catalytic cathode34 to that used in Examples 1 through 3. The electrodes had a catalystlayer that was 0.5-1 mm thick. Oxygen was the cathode chamber feed. Theoperating pressure in both of anode chamber 12 and cathode chamber 18was atmospheric pressure and the temperature of cell 10 was 88° C. Theopen circuit potential generated was 655 millivolts after operating for2 hours, which decreased to 644 millivolts after a further 2 hours. Whenan electrical load was connected across electrical leads 64, it wasfound that the current and the potential provided by the cell variedwith the load. When the external circuit load was 1.0 ohm, the potentialgenerated was 305 millivolts and the current generate was 305 milliampsafter operating for 2 hours. FIG. 4 is a diagram showing therelationship between current and potential for operation of a laboratoryscale version of the cell illustrated in FIG. 1 for the electrochemicaloxidation of butane to butene at atmospheric pressure and 90° C. whereinthe catalyst layer was at least 2 mm thick.

[0091] When the active catalyst of the catalytic anode was 2.4% platinumand laboratory cell 10 was operated at 88° C., the initial open circuitpotential was 494 millivolts, which decreased to 429 milllivolts afteroperating for 2 hours.

[0092] When the active catalyst of the catalytic anode was 9.1%palladium, the open circuit potential was 368 millivolts at an operatingtemperature of 90° C. Laboratory cell 10 was shut down after 3 hours,and then restarted after a further 19 hours. The open circuit potentialthen was 255 millivolts at an operating temperature of 82° C. and 392millivolts at 102° C.

[0093] When a thick layer (i.e. over 1 mm) of catalyst was used, theperformance was reduced, as illustrated in FIG. 4, and the maximum powerdensity was 11 milliwatts per square centimeter. No butyne was observed.

EXAMPLE 5

[0094] Nitrogen gas was passed through liquid ethyl benzene at 90° C. toform a feed gas stream. The gas stream comprising ethyl benzene innitrogen as an inert carrier gas was fed at 15 milliliters per minute asanode chamber feed to laboratory cell 10 having a catalytic anode 32comprising compressed Teflonized carbon powder loaded with palladium asfirst metal catalyst 36 and the same catalytic cathode 34 as was used inExamples 1 through 3. The electrodes had a catalyst layer that was 0.5-1mm thick. Oxygen was the cathode chamber feed. The operating pressure inboth of anode chamber 12 and cathode chamber 18 was atmospheric pressureand the temperature of cell 10 was 90° C. The open circuit potentialgenerated was 460 millivolts. When an electrical load-was connectedacross electrical leads 64, it was found that the current and thepotential provided by the cell varied with the load, as illustrated forone experiment in FIG. 5 wherein the catalyst layer was over 2 mm thick.When the external circuit load was 1.0 ohm, the potential generated was25 millivolts and the current generated was 25 milliamps. No ethynylbenzene was observed.

1. An electrochemical process for oxidation of an alkane to acorresponding alkene using an electrochemical cell having an anodechamber having an anode and a cathode chamber having a cathode, theanode chamber and the cathode chamber separated at least in part by aproton conducting medium, said process comprising: a. providing at leastone alkane to the anode chamber; b. providing an oxygen containing gasto the cathode chamber; c. passing protons through the said medium fromthe anode chamber to the cathode chamber whereby at least a portion ofthe alkane is converted to a corresponding alkene.
 2. Theelectrochemical process as claimed in claim 1 wherein the anodecomprises at least one metal catalyst active for activation of thealkane and the anode and cathode are in electrical contact with eachother and the process comprises producing electrons during theconversion of the alkane to the alkene and the catalytic cathodecomprises at least one metal catalyst active for combination of oxygenwith protons and electrons to form water.
 3. The electrochemical processas claimed in claim 1 further comprising maintaining the electrochemicalcell at a temperature and a pressure that maintains the moisture of saidmedium.
 4. The electrochemical process as claimed in claim 1 furthercomprising providing the alkane in a gaseous state.
 5. Theelectrochemical process as claimed in claim 1 wherein the alkane isselected from the group consisting of propane, a mixture of propane andat least one inert gas, a mixture of propane and at least one inertliquid, and a mixture of hydrocarbons containing propane, and theprocess comprises producing propylene as the corresponding alkene. 6.The electrochemical process as claimed in claim 1 wherein the alkane isselected from the group consisting of butane, a mixture of butane and atleast one inert gas, a mixture of butane and at least one inert liquid,and a mixture of hydrocarbons containing butane, and the processcomprises producing at least one of 1-butene and 2-butene as thecorresponding alkene.
 7. The electrochemical process as claimed in claim1 wherein the alkane is selected from the group consisting of a mixtureof ethyl benzene and at least one inert gas, a mixture of ethyl benzeneand at least one inert liquid, and a mixture of hydrocarbons containingethyl benzene, and the process comprises producing styrene as thecorresponding alkene.
 8. The electrochemical process as defined in anyone of claims 1-4 wherein the oxygen containing gas is selected from agroup consisting of oxygen, a mixture of oxygen and at least one inertgas, and air and the process further comprises combining protons whichhave passed through the medium and oxygen to produce water.
 9. Theelectrochemical process as defined in any one of claims 1-4 wherein theprocess is operated at a temperature of at least about 5020 C.
 10. Theelectrochemical process as defined in any one of claims 1-4 wherein theprocess is operated at a temperature in the range of about 5020 C. toabout 155° C.
 11. The electrochemical process as defined in any one ofclaims 1-6 in which the process is operated at a temperature in therange of about 50° C. to about 100° C.
 12. The electrochemical processas defined in claims 1-7 in which the process is operated at a pressureof at least atmospheric pressure and below a pressure at which one ormore of the alkane and the alkene will condense to form a liquid phase.13. The electrochemical process as defined in claims 1-7 in which thepressure is maintained sufficiently high so as to maintain moistness ofthe proton-conducting medium.
 14. The electrochemical process as definedin claim 5 in which the process is operated at a pressure of at leastatmospheric pressure and below a pressure at which one or more ofpropane and propylene will condense to form a liquid phase, the pressurebeing sufficiently high so as to maintain the moistness of the protonconducting medium at the operating temperature.
 15. The electrochemicalprocess as defined in claim 6 in which the process is operated at apressure of at least atmospheric pressure and below a pressure at whichone or more of butane, and at least one of 1-butene and 2-butene willcondense to form a liquid phase, the pressure being sufficiently high soas to maintain the moistness of the proton conducting medium at theoperating temperature.
 16. The electrochemical process as defined inclaim 7 in which the process is operated at a pressure of at leastatmospheric pressure and below a pressure at which one or more of ethylbenzene, and styrene will condense to form a liquid phase, the pressurebeing sufficiently high so as to maintain the moistness of the protonconducting medium at the operating temperature.
 17. The electrochemicalprocess as defined in any one of claims 1-7 in which the processis-operated at a pressure in the range of about 0.5 atm to about 10 atm.18. The electrochemical process as defined in any one of claims 1-7 inwhich the process is operated at about atmospheric pressure.
 19. Anelectrochemical apparatus for oxidation of an alkane to a correspondingalkene comprising: a) an anode chamber having an anode, the anodecomprising a metal catalyst active for activation of the alkane; b) acathode chamber having a cathode, the cathode comprising a metalcatalyst active for combination of a proton acceptor with protons; and,c) a proton conducting medium positioned in fluid flow communicationwith both the anode chamber and the cathode chamber.
 20. The apparatusas claimed in claim 19 wherein the proton acceptor comprises oxygen. 21.The apparatus as claimed in claim 19 wherein the proton acceptor is agas selected from a group consisting of oxygen, a mixture of oxygen andat least one inert gas, and oxygen is combined with protons that havepassed through the medium and oxygen to produce water.
 22. The apparatusas claimed in claim 19 wherein the alkane is gaseous.
 23. The apparatusas claimed in claim 19 wherein the alkane is a linear molecule or alinear substituent of a cyclic or aromatic molecule.
 24. The apparatusas claimed in claim 19 wherein the alkane has a carbon chain length offrom 2 to 6 carbon atoms.
 25. The apparatus as claimed in claim 19wherein the proton conducting medium is a solid perfluorosulphonic acidproton conducting membrane.
 26. The apparatus as claimed in claim 19wherein the catalytic anode and the catalytic cathode separately areformed of compressed carbon powder loaded with metal catalyst, the metalcatalyst of the catalytic anode being selected from metal catalystsactive for activation of an alkane, and the metal catalyst of thecatalytic cathode being selected from metal catalysts active forcombination of oxygen with protons and electrons to form water.
 27. Theapparatus as claimed in claim 19 wherein the alkane comprises propaneand the catalytic anode and the catalytic cathode separately are formedof carbon cloth loaded with metal catalyst, the metal catalyst of thecatalytic anode being selected from metal catalysts active foractivation of propane, and the metal catalyst of the catalytic cathodebeing selected from metal catalysts active for combination of oxygenwith protons and electrons to form water.
 28. The apparatus as claimedin claim 19 wherein the alkane comprises butane and the catalytic anodeand the catalytic cathode separately are formed of nickel meshimpregnated with metal catalyst, the metal catalyst of the catalyticanode being selected from metal catalysts active for activation ofpropane, and the metal catalyst of the catalytic cathode being selectedfrom metal catalysts active for combination of oxygen with protons andelectrons to form water.
 29. The apparatus as claimed in claims 19-28wherein the metal catalyst active for activation of alkane is selectedfrom the group consisting of platinum, palladium, silver, nickel,cobalt, gold, bismuth, manganese, vanadium, ruthenium, copper, zinc,chromium, iron or indium oxide-stannous oxide mixtures, or any mixturesthereof.
 30. The apparatus as claimed in claims 19-28 wherein the metalcatalyst active for activation of the alkane is selected from the groupconsisting of nickel, cobalt or a mixture of nickel and cobalt.
 31. Theapparatus as claimed in claims 19-28 wherein the metal catalyst foractivation of alkane is selected from the group consisting of platinum,palladium or a mixture of platinum and palladium.
 32. The apparatus asclaimed in claim 20 or 21 wherein the metal catalyst active forcombination of oxygen with protons and electrons to form water isselected from the group consisting of nickel, cobalt, gold, bismuth,manganese, vanadium, ruthenium, copper, zinc, chromium, iron or indiumoxide-stannous oxide mixtures, or any mixtures thereof.
 33. Theapparatus as claimed in claim 20 or 21 wherein the metal catalyst activefor combination of oxygen with protons and electrons to form water isselected from the group consisting of nickel, cobalt or a mixture ofnickel and cobalt.
 34. The apparatus as claimed in claim 20 or 21wherein the metal catalyst active for combination of oxygen with protonsand electrons to form water is selected from the group consisting ofplatinum, palladium or a mixture of platinum and palladium.
 35. Theapparatus as claimed in claim any one of claims 19-28 wherein theapparatus is operated at a temperature of at least about 50° C.
 36. Theapparatus as claimed in claim any one of claims 19-28 wherein theapparatus is operated at a temperature in the range of about 50° C. toabout 155° C.
 37. The apparatus as claimed in claim any one of claims19-28 wherein the apparatus is operated at a temperature in the range ofabout 50° C. to about 100° C.
 38. The apparatus as claimed in claim anyone of claims 19-28 wherein the process is operated at a pressure of atleast atmospheric pressure and below a pressure at which one or more ofthe alkane and the alkene will condense to form a liquid phase.
 39. Theapparatus as claimed in claim any one of claims 19-28 wherein thepressure is maintained sufficiently high so as to maintain moistness ofthe proton-conducting medium.